1. Field of the Invention
The present invention relates to a battery charger for charging rechargeable batteries such as nickel-cadmium batteries (hereinafter referred to as "Ni-Cad batteries").
2. Description of the Related Art
Various battery chargers for charging rechargeable batteries have heretofore been proposed. Universal battery chargers are particularly widely used. Universal battery chargers are capable of charging various types of batteries formed from different numbers of serially connected cells. That is, a user can charge any type of battery, regardless of the number of cells it contains, in the same universal battery charger.
There is a great demand for hi-powered battery-operated power tools. High-voltage batteries formed from many cells have been produced to power these hi-powered tools. Because universal battery chargers should be able to charge all batteries with the same current, universal battery chargers are being produced large enough and with output capability that is high enough to also cope with charging high-voltage batteries.
Because universal battery chargers are provided with capacity of supplying power required for charging high-voltage batteries, that is, batteries with many cells, universal battery chargers are not used to their full potential when recharging batteries with few cells. For example, a universal battery charger capable of recharging a 20-cell Ni-Cad battery at 7 A can supply a maximum power of 184.8 W (i.e., 1.32 V electromotive force per cell.times.20 cells .times.7 A=184.8 W). The same universal battery charger is theoretically capable of recharging a 10-cell Ni-Cad battery at 14 A (i.e., 184.8 W/1.32 V.times.10 cells=14 A). However, the relationship between the maximum current rates of components, such as diodes, high-frequency transformer, and FET, housed within the confined space of the universal battery charger, makes it so the same universal battery charger can actually recharge a 10-cell Ni-Cad battery with a charge current value of only about 10 A.
The size (volume) of the batteries increases with the number of cells forming the battery. As a result, the difference in temperature rise between different cells also increases. Therefore, when all batteries, regardless of the number of cells contained therein, are charged with the same current, cycle life of batteries tends to decrease by an amount that corresponds to the number of cells in the battery. That is, batteries with many cells will have a shorter cycle life than those with fewer cells.